Roll support device for continuous metallic strip casting

ABSTRACT

Support device for an assembly ( 14 ) of the casting rolls ( 11, 11 ′) of a continuous metallic strip casting line comprising a movable support at each axial end. Such supports ( 17, 17′, 19, 19 ′) are provided with a hydraulic bearing ( 13 ) to reduce friction during motion with respect to the assembly. Between the movable supports of one of the rolls ( 11 ) and the assembly is located a hydraulic actuator ( 18 ), which thrusts the first roll ( 11 ′) towards the second roll ( 11 ′) against a stop ( 16 ). Between the second roll ( 11 ′) and the assembly there are located magnetostrictive actuators which thrust the second roll ( 11 ′) against the first roll ( 11 ). The device has a joint provided with a housing ( 22 ) inside which a telescopic tube ( 21 ) for the supply of the cooling water to conduits present in the rolls may slide. The housing ( 22 ) is connected to the assembly ( 14 ) by means of a bellows ( 27 ) allowing for oscillation.

FIELD OF THE INVENTION

This invention relates to a support device of ingot mould rolls forcontinuous metallic strip casting, and particularly to afriction-reducing device for the rolls during their adjusting movement.It also relates to a method for controlling the distance between therolls constituting an ingot mould.

STATE OF THE ART

Metallic strips are normally produced starting from continuously castingots or slabs, which are reduced in thickness by a series ofsubsequent operations comprising the preforging, hot and coldlamination, together with other intermediate treatments, for exampleheat treatments.

These operating methods involve very expensive plants and notableexpenditure of energy.

Hence, for some time the tendency is that of reducing the plant andbusiness costs by casting products with thickness as close as possibleto that of the final product; consequently, following the introductionof continuous slab casting, the thickness of the latter is reduced fromthe conventional 200–300 mm to 60–100 mm obtained in the so-called “thinslab casting”. However, even the passage from 60 mm to 2–3 mm, which isthe typical thickness of a hot strip, requires a series of energeticallytaxing steps.

In view of the inherent disadvantages in casting bodies of significantthickness for reduction to thin strips the inherent advantages indirectly casting metallic strips have been recognised since the secondhalf of the 19^(th) Century, when Thomas Bessemer patented a machine forthe continuous casting of steel strip provided with a couple of cooledmetallic counter-rotating rolls set a small distance apart; the metalwas cast in the space between the rolls, solidified upon contact withthe cold surfaces of the latter and was finally extracted with athickness equal to the distance between the facing surfaces of the rollsthemselves.

Such extremely attractive technology has found practical uses for thecasting of metals such as copper and aluminium only in the last decadesof the 20th century, whilst for high smelting point metals and alloys,such as steel, at present the real industrial spread of such technologyis still not manifest.

Numerous efforts are made in this field essentially to reduce productioncosts, the energy consumed and the environmental impact, and to producethin strips directly usable just like they are, in particularapplications in which for example surface quality is not a particularrequirement, or to be considered the same as hot laminated strips forthese uses in which thickness' of less than a millimetre are necessary.

Being established that the machine conceived by Bessemer in his time isstill, in its general form, the most ideal for continuous metallic stripcasting, the problems to solve for its effective use are very numerousand range from ensuring the tightness of the rolls at their flat ends,to the most suitable materials to survive the demanding workingconditions, to the automated control of all the operations and thecasting speed and drawing of the strip, up to its winding into a coil.

One of the more stressed points along the line are the casting rolls,which normally must ensure, in the presence of high thermal stresses, aconstant quality of the cast strip and a suitable duration.

A characteristic of the continuous strip casting technology is that thestrip thickness depends on the roll rotation speed, under the samecasting conditions, such as steel solidification temperature, etc.

The casting rolls are one of the most complex parts of the casting line,since they must comprise, inter alia, a cooling system for the rollsthemselves, and a delicate support system, which must also allow for,inter alia, the cast strip thickness adjustment. These requirementsinvolve the presence of a number of elements implementing the variousfunctions required by the plant. A solution adopted in known plants isarranging the rolls together with the devices performing many functions,directly related to their operation, such as the cooling system and theroll distance control in a complex assembly platform that allows fortheir quick replacement either in case of routine or extraordinarymaintenance.

A continuous strip casting plant with a casting roll support platformcomprising a complex system of roll supports is known from EP-A-903190and EP-A-903191. In order to favour the displacement of the rolls duringtheir side movements of removal and approaching during the casting, insuch plant a linear bearing system is also provided.

A problem to be solved in the plants of this type is that of ensuringmovements of approaching and removal which are as quick as possible alsoto face emergency conditions, such as when a quick and almost immediatedistancing of the rolls is required to drop the molten metal which isstill upon the rolls.

Another problem to be solved is that of improving the reliability of thesupports to minimize the danger of seizure in operation, which maycompromise the roll assembly itself with serious consequences.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to solve theabove-mentioned problems by providing a support device which ensures themaximum reliability during the continuous metallic strip casting, anevenly thick strip, and which can be used in the presence of hightemperature, and allows for the required displacement of the rolls withcoordinate and sufficiently precise movements of the supports of the twoopposite ends of each roll, to avoid lack of symmetry or planaritydefects of the strip thickness. Such problems are solved according toclaim 1 by support device on a assembly of a first and a second cooledcasting rolls with a pair of plates abutted on each end of said pair ofrolls, working as a mould for continuous metal strip casting, said firstand second rolls having parallel axes and each of them being supportedby at least one movable support element near to the axial ends, saidmovable support elements being suitable for allowing a mutual movementof approaching and distancing of said rolls of said pair, each movablesupport element associated with the first roll being connected to saidassembly by means of its respective hydraulic actuator suitable forthrusting said first roll in the direction of said second roll andsuitable for thrusting each support element against an abutting endelement, each movable support element associated with the second rollbeing connected to said assembly by a magnetostrictive actuator and aload cell suitable for making said second roll perform movements ofmutual approaching and distancing from said first roll, wherein a bar ofthe magnetostrictive actuator is provided with a preloading system andbetween each movable support element and said assembly there is providedat least one respective hydraulic bearing suitable for allowing slidingmovement of each of said movable support elements with respect to saidassembly.

Preferably, said assembly is the frame of a box containing the castingrolls and the other assemblies stated above.

Owing to the innovative characteristics of the present invention theroll supports, made by providing the hydrostatic bearings ensuring afluid film between the rolls themselves and the support platform, reducea lot the friction coefficient in the support. Such solution allows oneto obtain better results also for use near to a heat source at a hightemperature.

The casting process is kept to an optimal level thanks to thecharacteristics of the supports, which during the movement in thedirection of mutual distancing and approaching of the rolls present aminimum friction both between the roll supports and the stationary frameof the box and between the joint for the feeding and draining the rollcooling water. The friction minimization obtained with the supportdevice of the present invention is also important to ensure a symmetricprocess, otherwise different conditions can occur to the same roll withtwo different supports and the cast strip will consequently have avariable thickness along its width.

According to the a further aspect of the present invention, suchproblems are solved according to claim 7 by a method for controlling andadjusting the axial distance of the casting rolls for a continuousmetallic strip casting implemented with the device of claim 1 comprisingthe following stages:

-   (a) operating said hydraulic actuator to make a first roll approach    in the direction of the second roll until at least one respective    movable support element associated with the first roll is in close    contact against an abutting end element.-   (b) emitting control signals to a magnetostrictive actuator    depending on the signals received relevant to suitable process    parameters;-   (c) operating the magnetostrictive actuator to apply a force onto    the movable supports associated with the second roll in the    direction of a mutual approaching to or of a distancing from the    first roll by sliding on at least a respective hydraulic bearing    depending on the intensity variation of the roll separation force,    so that the minimum gap between the rolls is kept constant.

LIST OF THE DRAWINGS

Further advantages obtainable with the present invention will be moreevident to those skilled in the art by the following detaileddescription of a particular non-limiting embodiment of a support devicefor continuous metallic strip casting rolls with reference to thefollowing Figures in which:

FIG. 1 schematically shows a section in a vertical plane of a metallicstrip casting line;

FIG. 2 schematically shows an axonometric view of a roll support box;

FIG. 3 schematically shows a section of the roll support device of theinvention;

FIG. 4 shows a section of a joint for the supply of the cooling fluid tothe rolls being a part of a device according to a preferred aspect ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the above-mentioned Figures, the continuous castingdevice provides for a ladle 1 which unloads the liquid steel loadthrough an unloading slide valve 2 and a conduit 3 into a tundish 4.From the latter, the steel passes through a further conduit 5 into anoptional under-tundish, not shown, or through an unloading device intoan ingot mould 10 comprised of a pair of counter-rotating cooled castingrolls 11, 11′, turning around their respective and mutually parallelaxes X, X′. Two bulkheads indicated by the reference numeral 30 in FIG.2 are provided to complete the ingot mould 10 and restrict the liquidmetal in the direction of the roll axes between the rolls themselves bysuitable means which thrust them against the roll end surfaces.

In such ingot mould 10, the liquid metal solidifies in contact with saidrolls 11, 11′ and is extracted from the ingot mould in the form of astrip at high temperature, said strip following, below said ingot mould,by gravity a substantially vertical path 12. The rolls and a number ofother devices associated with them are arranged in a box 7, which ispartially shown in greater detail in FIG. 2. Here a portion of the frame14 of the box, particularly the bottom and the assemblies contained inthe box, is shown.

The roll support device according to the present invention is shown indetail, by way of a non-limiting example of the scope and object of theinvention in FIGS. 2 and 3 in a possible embodiment thereof.

The casting rolls 11, 11′ are mounted on four support elements 17, 17′,19, 19′ preferably two for each roll, which in turn rest on the frame 14of the box 7. Between the support elements 17 and the frame 14 of thebox 7 some hydraulic bearings 13, 13′ are provided one of which ispreferably provided near to each support element 17, 17′. During thecasting operation, the roll 11 is kept stationary by pushing eachsupport 17, 19 of the roll 11 against a stop 16, by means of one or morehydraulic cylinders 18, 18′ preferably two, which push it towards thesecond roll 11′.

This roll 11 is conventionally known as “stationary” because during thecasting operation it rests against the abutting end 16, while the otherroll 11′ is known as “movable” because, during the casting operations,it is the one which performs the operations necessary to a correctcasting operation. The roll 11′ is pushed towards the roll 11 by meansof at least one magnetostrictive actuator 15 which in an advantageousembodiment are preferably two 15, 15′ arranged each at each roll end.The cylinders 18, 18′ are connected to the stationary roll 11 and themagnetostrictive actuators 15, 15′ are connected to the movable roll 11′with their respective first end and are fixed to their second respectiveend to the frame 14 of the box 7, for example to the sides thereof,which are not shown in FIG. 3 to allow for a better view of the system.

The magnetostrictive actuators are devices based on the intensemagnetostrictive effect of some metallic alloys. Such materials arecapable of elongation, the so-called negative magnetostriction, in thedirection of a magnetic field applied thereto. They are also capable tovary the orientation of the magnetic domains as a consequence of thecompression or traction to which they are subjected.

Enhancement of the magnetostrictive effects occurs in the iron and rareearth alloys, such as samarium, terbium, dysprosium, etc. Such effect ismaximum when the magnetic field reaches the saturation valve of thematerial. Furthermore, it ceases once the Curie temperature is attained.In the Table 1 below the main features of some magnetostrictivematerials are listed, which are particularly suitable for use in theconstruction of magnetostrictive actuators.

Saturation Magnetostriction Curie temperature Material [μm/m] [° C.]SmFe₂ −2100 402.85 TbFe₂ 2460 424.56 DyFe₂ 1260 362 HoFe₂ 200 333 ErFe₂−300 317.45 TmFe² −210 287 Fe −9 770 Ni −33 354 CoFe₂O₄ −110 —

As such magnetostrictive materials are quite fragile, a preload systemof the bar made of such material is suitably provided in the actuatorsto prevent the bar from being stressed by traction during operation withdamaging consequences.

Such actuators offer optimal characteristics of use, among which thereis the hihgh frequency good response in addition to the short reactiontime and the high force applicable. For example, one of themagnetostrictive alloys presents an optimal frequency interval of 0–5kHz, furthermore a bar in such material, 10 cm long, can elongate ofmore than 0.1 mm in 50 μs and a bar with a diameter of 30 mm can bear aforce of 2 tons.

In the support device bearings 13, 13′ are provided to reduce thefriction coefficient during the movement of the casting rolls 11, 11′ inthe direction of mutual approaching and distancing of their axes X, X′.Such movements of the rolls 11, 11′ which must be performed whilekeeping the parallelism between their axes X, X′ with the utmostaccuracy, have the purpose of controlling the thickness of the caststrip. The bearings used are advantageously of the hydrostatic type asshown in detail in FIG. 3. In this way, between the supports 17, 17′,19, 19′ of both the movable and stationary rolls, and the frame 14 ofthe box 7 there is a fluid film which dramatically reduces the friction.

The operation of the support device of the pair of rolls according tothe invention is described herewith below for one magnetostrictiveactuator only, it is however understood that the second support of theroll at the second end of the pair of rolls also has the same technicalfeatures and operates in the same way. In the event that during thecasting process the strip production speed, or some other castingparameter, such as the superheat of the liquid steel, is altered, theroll 11′ may approach or distance itself from the roll 11, to keep theseparation force of the roll themselves quite constant, thus ensuringconstant working conditions, and particularly that the solidificationcomplexion point remains the same, preferably near to the so-called“KISSING POINT” (KP).

When the separation force of the casting rolls 11, 11′ begins to change,this means that the solidification point moves away from the KP point.In this case, the position of the movable roll 11′ must change to makethe separation force go back to the pre-established value by moving themovable roll in closer to or further away from the stationary roll, andthis results in keeping the solidification complexion point near to theKP point.

In order to adjust the position of the movable roll 11′, themagnetostrictive actuator 15 is connected to the support 17 of themovable roll 11′, and a load cell is also provided between them. Thesame applies to the second end of the movable roll 11′ driven by thesecond magnetostrictive actuator 15′. The magnetostrictive bar 15 a ispreloaded with a suitable preloading system 15 c and the initialposition of the movable roll 11′ is ensured by a position transducer. Inthe initial position, the magnetostrictive bar is elongated by apre-established value under the action of the magnetic field produced byelectric coils 15 b and this ensures the support 17′ being thrustagainst the cast strip.

As soon as the intensity of the roll separation force varies, thecontrol system varies the intensity of the magnetic field either toelongate or shorten the magnetostrictive bar as a function of thevariation of the separation force, and as a result the positioning ofthe rolls is also varied in such a way that, by keeping the forceconstant, the complexion of the solidification at KP point is alsoensured. The response of the system is very quick since the distancebetween the rolls can be varied in some tens of μs.

In an advantageous embodiment of the invention, the support devicepreferably further comprises one or more connection joints for conduitsof the cooling liquid for the rolls, which are globally indicated by thereference numeral 20. One of these is schematically shown in FIG. 4.Cooling is required to keep the surface temperature of the rolls 11, 11′as constant as possible, by dissipating the metal solidification heat.Given the considerable amount of heat to be dissipated, the conduits ofthe cooling liquid must be duly sized. The cooling system must alsoallow for the mutual approaching and distancing movements of the rolls11, 11′, whether they are small, for example when varying the stripthickness, or big, when distancing the rolls 11, 11′, for example inorder to empty the ingot mould 10 of the liquid steel contained therein.

The joint 20 comprises a telescopic tube 21 arranged substantiallyhorizontally, and in which liquid conduits are inserted both in thefeeding direction to the rolls, and in the outlet direction from therolls after the cooling. Preferably, there are provided two joints pereach roll 11, 11′ which are located at each end of each roll, one forfeeding the liquid to the roll and the other for taking the liquid awayfrom the roll. The telescopic tube 21 is coaxially inserted in a housing22 provided with suitable gaskets 23, 24, which allow for the axialsliding displacement of the tube 21 in the housing 22 in case of bigdisplacements of the rolls 11, 11′. Such displacements can be performedin emergency conditions by means of hydraulic cylinders arranged near toeach support 17, 17′, 19, 19′, which in case of the movable roll 11′ arearranged in series with the magnetostrictive actuator.

As can be seen from the FIG. 4, which shows one of the four rollsupports 11 and 11′, since the other three supports are made in the sameway, consequently the bellows or compensator 27 allows the roll 11 toperform small displacements of the rolls during the casting in thedirection of the arrows 28, 28′ even if the axial sliding of the tube 21in the housing 22 does not take place and correspond to smalldisplacements of an oscillatory type of the joint 20 in the direction ofthe arrow 29 during the operation of the casting machine. Such movementsmust take place with as little friction as possible and the presence ofthe vertical bellows 27 allows for it, and they are recovered with noresistance while the axial sliding of the tube 21 would involve greaterdissipation.

If big displacements are required, of the same type as those envisagedwhen opening the rolls for the emergency evacuation of the metal presenttherebetween in the ingot mould, the bellows or compensator 27 of the“stationary” roll 11 allows the housing 22 to make a first displacementuntil coming into tight contact with one of the stops or abutting endelements 25 and does not suffer from distortions which may damage it andsubsequently the axial sliding of the tube 21 takes place, which allowsfor the axial distancing of the rolls. Both the stationary roll 11 andthe movable roll 11′ are opened in the same way.

Other bellows can be advantageously provided around the tube 21 forexample in order to protect if from dust or other foreign elements.There are also provided support and gasket elements 23 and 24 comprising“O-ring” thereby ensuring the sealing from the water flowing between thetube 21 and the housing 22.

The cooling water flows in the vertical direction, for example in thedirection of the arrow 31 in the tube comprising the vertical bellows27, then it passes through holes, not shown in the figures, in thehorizontal telescopic tube 21 and subsequently in the respective castingroll 11, 11′. The water, after having performed its cooling function,follows the path in reverse and passes from the roll 11, 11′ to thetelescopic pipe 21, then through holes in the vertical tube comprisingthe bellows 27.

By means of said joint for the conduits of water, or any other type ofcooling liquid which is adapted to perform such a function, the globalresistance of the support device in relation to the displacementscommanded to the rolls 11, 11′ and this presents the advantage that thedistance between the rolls is self-regulated in a very precise manner,for example according to the casting speed, and that the strip thicknessis even all along its width. Excessive friction in the supports, infact, may compromise the integrity of the strip thickness uniformity.

1. A support device on a assembly (14) of a first and a second cooledcasting rolls (11, 11′) with a pair of plates (30, 30′) abutted on eachend of said pair of rolls (11, 11′), working as a mould (10) forcontinuous metal strip casting, said first and second rolls (11, 11′)having parallel axes (X,X′) and each of them being supported by at leastone movable support element (17, 17′, 19, 19′) near to the axial ends,said movable support elements (17, 17′, 19, 19′) being suitable forallowing a mutual movement of approaching and distancing of said rolls(11, 11′) of said pair, each movable support element (17, 19) associatedwith the first roll (11) being connected to said assembly by means ofits respective hydraulic actuator (18, 18′) suitable for thrusting saidfirst roll (11) in the direction of said second roll (11′) and suitablefor thrusting each support element (17, 17′) against an abutting endelement (16), each movable support element (17′, 19′) associated withthe second roll (11′) being connected to said assembly by amagnetostrictive actuator and a load cell suitable for making saidsecond roll (11′) perform movements of mutual approaching and distancingfrom said first roll (11), wherein a bar of the magnetostrictiveactuator is provided with a preloading system and between each movablesupport element (17, 17′, 19, 19′) and said assembly (14) there isprovided at least one respective hydraulic bearing (13, 13′) suitablefor allowing sliding movement of each of said movable support elements(17, 17′, 19, 19′) with respect to said assembly (14).
 2. The deviceaccording to claim 1, wherein there is provided at least one joint (20)supporting a cooling liquid conduit between said rolls (11, 11′) andsaid assembly suitable for allowing for mutual approaching anddistancing of said rolls (11, 11′) in an orthogonal direction to theaxes.
 3. The device according to claim 2, wherein said joint (20)comprises a telescopic tube (21) inserted substantially horizontally ina housing (22) connected to said assembly, said tube being suitable forsliding along its own axis in said housing.
 4. The device according toclaim 3, wherein a bellows or a compensator (27) is set between saidassembly and said housing.
 5. The device according to claim 4, whereinan abutment means (25, 26) is provided to limit the displacements ofsaid housing in the direction of the axis of said telescopic tube (21).6. A method for controlling and adjusting the axial distance of thecasting rolls (11, 11′) for a continuous metallic strip castingimplemented with the device of claim 1 comprising the following stages:a) operating said hydraulic actuator (18, 18′) to make a first roll (11)approach in the direction of the second roll (11′) until at least onerespective movable support element (17, 19) associated with the firstroll (11) is in close contact against an abutting end element (16). b)emitting control signals to a magnetostrictive actuator depending onsignals received relevant to suitable process parameters; c) operatingthe magnetostrictive actuator to apply a force onto the movable supportselements (17′, 19′) associated with the second roll (11′) in thedirection of a mutual approaching to or of a distancing from the firstroll (11) by sliding on at least a respective hydraulic bearing (13,13′) depending on the intensity variation of a roll separation force, sothat a minimum gap between the rolls (11, 11′) is kept constant.
 7. Themethod according to claim 6, wherein a control system varies theintensity of magnetic fields either to elongate or shortenmagnetostrictive bars comprised in said actuation means (15) as afunction of the intensity variation of the separation force.
 8. Asupport device on an assembly of first and second cooled casting rollswith a pair of plates abutted on each end of said pair of rolls, workingas a mould for continuous metal strip casting, said first and secondrolls having parallel axes and each roll being supported by at least onemovable support element, said movable support elements being operativefor allowing movement of approaching and distancing of said rolls, eachmovable support element associated with the first roll being connectedto said assembly, each movable support element associated with thesecond roll being connected to said assembly by a magnetostrictiveactuator operative to permit incremental movement of approaching anddistancing between the first and second rolls.
 9. The device accordingto claim 8 further comprising a control system operative to vary theintensity of magnetic fields about a magnetostrictive bar of theactuator to either elongate or shorten the magnetostrictive bar as afunction of the separation force.